1. Field
Embodiments of the present disclosure generally relate to methods of measuring reflected power from a plasma chamber using a directional coupler.
2. Description of the Related Art
In RF plasma processing systems, RF power is typically coupled into the plasma that is present in the reactor chamber through a match circuit which contains variable reactance elements. The purpose of the match circuit is to transform the impedance of the load (i.e., the plasma) to a value that equals or matches the characteristic impedance of the transmission line through which the RF power is delivered to the reactor chamber. At the match point, optimum power is delivered into the plasma and zero (or near zero) power is reflected back toward the RF generator.
However, the impedance of the load tends to vary during processing of the substrate, due to fluctuations of the plasma in the reactor chamber. In case of unstable plasma, such as arcing, a fast interruption or spike will occur in RF voltage and RF current and cause the impedance of the load to change abruptly, which in turn causes a spike in the RF power reflected back to the RF generator and therefore an RF impedance mismatch. As RF impedance mismatch occurs, the amount of RF power that is reflected back to the RF generator increases, while the amount of RF power delivered to the plasma decreases. Such fluctuations change the plasma conditions and therefore affect the plasma processing of the substrate, making it difficult to control process parameters such as etch rate or deposition rate.
Therefore, improved arc detection apparatus and methods are needed to monitor RF power reflected back from the reactor chamber for better plasma processing of the substrate.